The achievable gains of MIMO NOMA at lower frequencies are disputed, citing equivalent or superior performance provided by the much simpler multi-user linear-precoding (MU-LP) scheme [1]}. MU-LP is a special class of broadcasting techniques that, unlike NOMA, do not employ SIC at any receiver. That is, a superposition signal is transmitted, and each user recovers its own data by treating other users signals' as noise.
The multiplexing gain, which measures how fast the rate changes with SNR (system performance at high SNR), is studied for NOMA and MU-LP in [1]}. NOMA achieves lower multiplexing gains than MU-LP under full-rank channels due to the SIC rate constraint in the latter. In particular, the NOMA transmission rate is restricted to support both paired users while preventing SIC error, which does not apply to MU-LP. This argument assumes a zero-forcing beamformer (ZFBF) to mitigate intra-cluster interference.
